The present invention relates to voltage-controlled amplifiers (hereinafter referred to as VCAs), in which the gain is controlled through voltage control, and more particularly, to VCAs with improved gain control accuracy.
FIG. 5 shows the circuit construction of a prior art VCA. In this VCA, the gain of input signal Vin is controlled by varying control voltage Vc, thus providing output signal Vout. Referring to the Figure, the illustrated VCA comprises pair transistors Q1 and Q2, Q3 and Q5, Q4 and Q6, Q7 and Q8, Q9 and Q10, and Q11 and Q12, having the same characteristics, pair resistors R1 and R2, and R6 and R7, having the same resistance, and pair diodes D1 and D2, having the same characteristics. A difference voltage between the input signal Vin and a reference voltage Vref, is converted by a constant current source 4, the resistors R1 and R2 and the transistors Q1 and Q2 to a difference current, which is reflected by a current mirror which is constituted by the transistors Q3 to Q6. The difference current is divided and re-combined through the transistors Q7 to Q10. The amplitude of the difference current in the resistors R3 and R4 is changed according to the ratio of the division and re-combination. This change is controlled by the difference between base potentials on the transistors Q7 to Q10. This base potential difference is provided by a voltage Vc through a circuit, which is constituted by the transistors Q11 and Q12, the diodes D1 and D2, the resistors R5 to R7 and a constant current source 6. The circuit constituted by the transistors Q11 and Q12, the diodes D1 and D2, the resistors R5 and R7 and the constant current source 6, has the roles of causing circuit gain changes in proportion to the control voltage Vc and also providing a wide range of the voltage Vc. Where these roles are unnecessary, the voltage Vc may be directly applied to the bases of the transistors Q7 to Q10.
FIG. 6 shows the relation between the gain and the control voltage Vc in the above VCA. As shown, the gain is zero when Vc is zero, and increases with increasing Vc. When it is desired to use this VCA with a gain range of 8.+-.2, the maximum gain may be set to 10, and the range of the voltage Vc may be set to be between 6 and 10 as shown as zone b in FIG. 6.
The above VCA, however, is an open-loop amplifier, and is therefore subject to differential current errors due to such adverse effects as those due to VBE characteristics of the differential pair transistors Q1 and Q2, Q7 to Q10, and Q11 and Q12, stemming from the use of these transistors in an unbalanced state, and also those due to early transistor voltages, stemming from the conversion of the input signal to the differential current through the current mirror circuit. The differential current errors deteriorate the linearity characteristics and temperature characteristics. Changes in the linearity and temperature characteristics may lead to a pronounced gain change .DELTA.G2 particularly in the maximum gain side, as shown by the phantom plot in the characteristic in FIG. 6.
FIG. 7 shows a different VCA, which is described in Japanese Patent Disclosure No. 58-202606. Referring to the Figure, the illustrated VCA comprises transistors Q7 to Q10 and Q13 to Q18, resistors R8 to R14, constant current sources 10 to 13 and an operational amplifier 14. Although not described in detail, like the VCA described before, in this circuit the gain of Vout can be controlled through control of Vc. In this VCA, however, Vc is directly applied to the bases of the transistors Q7 to Q10. Thus, although the proportional relation between the gain and Vc as shown in FIG. 6 can not be obtained, the gain can be increased by increasing Vc. In this VCA, an intermediate gain is obtained when Vc is zero. Again in this VCA, the transistors Q7 to Q10 are used in an unbalanced state, and the gain is controlled after conversion of the input signal to the differential current. Therefore, like the VCA shown in FIG. 5, the linearity and temperature characteristics are deteriorated. Particularly, where a VCA is used for the gain correction preceding and succeeding amplifiers, high accurate linearity and temperature characteristics are required for accuracy improvement which is aimed at. With the prior art VCAs, however, it is difficult to meet the above requirements.
An object of the present invention is to improve the linearity and temperature characteristics and provide a VCA which can ensure high gain control accuracy.
According to a first aspect of the present invention, there is provided a voltage-controlled amplifier circuit comprising a voltage-controlled amplifier unit for amplifying an input signal with a gain variable according to a controlled voltage, a basic amplifier unit for amplifying the input signal with at least one fixed gain, and an adder for adding together the output signals of the voltage-controller amplifier unit and the basic amplifier unit.
The gain of the voltage-controlled amplifier unit is variable both positively and negatively from zero at the center of a range, the positive and negative gain ranges being each set to be less than the fixed gain of the basic amplifier unit. The voltage-controlled amplifier unit includes a differential input unit for converting a differential voltage between the input signal and a reference signal to a differential current, a current mirror circuit for transmitting the differential current, a differential amplifier for amplifying the differential current transmitted from the current mirror circuit, and a gain control unit for controlling the gain of the differential amplifier by converting a voltage for controlling the gain to a differential current. The basic amplifier unit includes an operational amplifier for amplifying the differential voltage between the input signal and the reference signal with a fixed gain, the reference signal being commonly used as a reference signal applied to the voltage-controlled amplifier unit. The output of the voltage-controlled amplifier unit is converted to a current output coupled to a resistor connected to the output side of the basic amplifier unit, the resistor serving as the adder.
According to another aspect of the present invention, there is provided a voltage-controlled amplifier circuit comprising: a basic amplifier unit for amplifying an input signal with a predetermined gain to the output signal; a voltage-controlled amplifier unit for amplifying the input signal with a predetermined gain to the output signal and providing a gain variable; and an adder for adding the output signals of the basic amplifier unit and the voltage-controlled amplifier unit and providing the resultant sum signal as a voltage-controlled amplifier output signal, the gain being variable in a range of G1 to G2, the gain of the basic amplifier unit being set to mid point gain G0 between G1 and G2, and the gain of the voltage-controlled amplifier unit being made variable between G1 and G2 with the gain G0 of the basic amplifier unit as the center gain.
The basic amplifier unit amplifies the difference or differential voltage between the input signal Vin and a reference voltage Vref as an input voltage with a predetermined standard gain, and provides the amplified output to the adder, the voltage-controlled amplifier unit includes a differential input unit for converting the differential voltage between the input signal Vin and a reference voltage Vref to a differential current, a current mirror circuit for transmitting the differential current, a differential unit for converting a gain controlled voltage to a differential current, and a gain control unit for controlling the gain according to the output of the differential unit.
Other objects and features will be clarified from the following description with reference to attached drawings.